The development of means of communication utilizing radio waves is remarkable. For example, while the UHF bands up to the frequency 300 MHz are currently used in wireless LANs and cellular phones, high-frequency LAN, satellite communication and cellular phone base stations, which will be the central targets of future development, will use microwave bands in the range of 3 to 30 GHz and there is a tendency for higher frequency bands to be used with the increase in amount of information.
In the case of using high frequency bands, fluororesins, which are excellent in electrical characteristics, namely stably low in dielectric constant [∈r] and low in dielectric loss constant [tan δ], have been regarded as appropriate and used as insulating materials or dielectric materials for use in connectors or like parts for electrical products, or in communication apparatus such as coaxial cables.
Known as fluororesins used in communications apparatus are tetrafluoroethylene homopolymers as well as such melt-processable polymers as tetrafluoroethylene [TFE]/hexafluoropropylene [HFP] copolymers [FEP; HFP content not lower than 6.9 mole percent] and TFE/perfluoro(alkyl vinyl ether) [PAVE] copolymers [PFA; PAVE content not lower than 1.5 mole percent].
However, when tetrafluoroethylene homopolymers are used, molding/processing tends to become difficult and, when melt-processable polymers are used, there is a problem, namely the electrical characteristics tend to deteriorate. Therefor, it has become necessary that fluororesins have good moldability/processability as well as good electrical characteristics; thus, various fluoropolymers have been proposed.
A non-melt-processable modified PTFE fine powder obtainable by copolymerizing TFE and a very small amount of a PAVE or a fluoroolefin has been disclosed as one requiring only a low extrusion pressure and showing great strength of bonding to wires (cf. e.g. Patent Document 1: Japanese Kohyo Publication H11-509245). However, this modified PTFE fine powder has a problem, namely it is inferior in electrical characteristics.
Known as ones improved in electrical characteristics are a TFE-based resin as a molding material which is a copolymerization product derived from TFE and a very small amount of a perfluoromonomer and has a core-shell structure (cf. e.g. Patent Document 2: Japanese Kokai Publication 2001-288227), and a TFE-based resin as a molding material which has a reduced dielectric loss tangent value as a result of molecular weight reduction (cf. e.g. Patent Document 3: Japanese Kokai Publication 2002-47315), respectively. However, these documents have neither descriptions nor suggestions about the relationship between the use of a chain transfer agent and the moldability/processability of the resulting resins.
An aqueous dispersion containing colloidal particles of a TFE polymer having a three-layer structure composed of a particle core, an inner particle covering layer and an outer particle covering (cf. e.g. Patent Document 4: Japanese Kokai Publication S56-92943). While this document describes that propane, methanol or the like can be used as a chain transfer agent, it is a problem that moldability/processability and electrical characteristics cannot coexist.